Spraying method and a spray system for coating liquids

ABSTRACT

The invention relates to a spray system and a spraying method for coating liquids. An accessory liquid ( 10 ) wets the front, external surface&#39;s terminal zone ( 8 ) of the spray system ( 2 ) discharging the spray jet ( 6 ) of coating liquid, for instance the front terminal zone of the external periphery of a rotating atomizing bell ( 4 ), in order to prevent drying and layering of coating liquid ( 10 ) precipitated on said external surface.

[0001] The invention relates to a method for spraying and to a systemfor spraying a coating liquid as defined in the preamble of claim 1 andclaim 8.

[0002] Spray systems comprising a rotary atomizing element in the formof a so-called bell to atomize and spray coating liquids onto an objectto be coated are known from the U.S. Pat. Nos. 4,275,383 and 4,505,430;German patent documents 30 00 002 A1 and 35 09 874 A1. They discloseapplying a high electrical potential, which may be positive or negative,to the rotary atomizers and/or to the spray coating liquid. Typicallythe high voltage is in the range of 4 kv to 140 kv. A high-voltage spraysystem fitted with an irrotational spray nozzle is known from U.S. Pat.No. 3,731,145.

[0003] Some spray particles of the sprayed coating material tend tomigrate rearward from the spray jet and to precipitate on the frontterminal outer surface zone of the spray system where they will cure andconstitute a hard coating.

[0004] The objective of the invention is to prevent or at least reduceadhesion and curing/hardening and the ensuing coating, i.e. layering ofthe coating material on the front terminal outer surface zone of thespray system.

[0005] This goal is attained in the invention by means of the featuresof claims 1 and 8 resp.

[0006] Accordingly the invention concerns a method for spraying coatingliquids wherein such a coating liquid is sprayed from a liquid atomizerin the form of an irrotational nozzle or preferably in the form of arotating, rotary atomizing element onto an object to be coated, and saidmethod is characterized in that the terminal zone of an outer surface ofthe spray system delivering the spray jet of coating liquid is wettedwith an accessory liquid in order to prevent or reduce layering ofcoating liquid precipitated on said surface.

[0007] The invention also concerns a coating-liquid spray systemcontaining a liquid atomizer in the form of an irrotational nozzle orpreferably in the form of a rotating rotary atomizing element to spraycoating liquid onto an object to be coated, characterized in that itcomprises a wetting unit to wet a front, terminal zone of the externalsurface of the spray system during spraying with an accessory liquid inorder to prevent or delay drying and layering of coating liquidprecipitated on said surface.

[0008] The invention offers the advantage that the accessory liquidprevents or strongly delays drying and layering of coating liquid on thespray system.

[0009] The wetting unit to wet the outer surface's front terminal zonewith said accessory liquid preferably contains one or more boreholes ornozzles through which the accessory liquid—which illustratively is inthe form of liquid drops or of an aerosol—is deposited in metered manneron the pertinent system component, for instance on rotating liquidatomizers.

[0010] In a further feature, the invention contains a cooling unit tocool at least part of the spraying system. Because of loweredtemperature, drying or layering of the coating liquid on the outersurface's front terminal zone shall be prevented or much delayed.Accordingly cooling the outer surface's terminal zone enhances theeffectiveness of the accessory liquid, namely to prevent or reducedrying and layering of coating liquid on said surface.

[0011] Preferably a cooling unit to cool the coolant shall be integratedinto the spraying system. Preferably a compressed gas, for instancecompressed air, shall be used as the coolant and it is cooled by thecooling unit and blown onto the cooling portion of the spraying system.

[0012] The voltage applied to the spraying system may be higher thanground or another predetermined reference potential on part or all ofsaid system. The higher voltage may be constant or variable and it maybe positive or negative relative to the reference voltage.

[0013] The invention is illustratively elucidated below by means of apreferred embodiment and the attached drawings.

[0014]FIG. 1 is a schematic sideview, also shown partly in longitudinalsection, of a spray system of the invention, and

[0015]FIG. 2 is a front elevation of the left, front side of the spraysystem of FIG. 1.

[0016] The coating-liquid spray system 2 shown in FIGS. 1 and 2 containsa liquid atomizer in the form of an atomizing bell 4 driven by anomitted drive—preferably a compressed air turbine—at an angular speedhigh enough to spray the coating liquid in the form of a spray jet 6onto an object to be coated. The coating liquid flows through the frontside 5 of the atomizing bell 4.

[0017] The external surface's peripheral front terminal zone 8 of theatomizing bell 4 is wetted during spray coating with an accessory liquid10. The accessory liquid 10 prevents or delays drying and layering ofcoating liquid that migrated back from the sprayjet 6 and precipitatedon the external surface's front, peripheral zone 8. The accessory liquid10 is metered through one or more discharge apertures 18 of a wettingunit 16 and is deposited on the external surface's front, peripheralterminal zone 8, for instance being applied drop-wise or in the form ofan aerosol, for instance being applied as a spray. A liquid feed line 20of the discharge apertures 18 is connected 20 through an external supplyline 22 containing a valve 24 to an accessory-liquid container 26. From-a compressed air source 30, compressed air is fed in controlled mannerthrough a valve system 28 to the container 26 where it drives theaccessory liquid out of said container 26 toward the spray system 2.

[0018] Preferably the accessory liquid shall be water when the coatingliquid is water soluble, or it shall be a solvent when the coatingliquid contains solvents.

[0019] As shown by FIG. 2, several, for instance three accessory liquidnozzles are configured as discharge apertures 18 across the externalsurface's front, terminal zone 8 in an air hood 32.

[0020] The feed system feeding coating liquid in the spray system to theatomizing bell 4 is omitted from the Figures because being known in thestate of the art, for instance that of U.S. Pat. Nos. 4,275,838 and4,505,430. Preferably a high voltage is applied to the atomizing bell 4,both the high voltage source and its connections being omitted form theFigures because being widely known.

[0021] To preclude sparkover from the spray system 2 onto the container26 of the accessory liquid 10, both said container and the atomizingbell 4 shall be kept at the same high voltage and are configured on abench 38 which is electrically insulated from the ground potential 34 byelectric insulators 36.

[0022] Preferably the preferred embodiment also shall contain a coolingunit 40 to cool the front parts of the spray system 2, in particular tocool the front side 5 over which flows the coating liquid and to coolthe front terminal zone 8 of the external surface of the atomizing bell4. The cooling unit 40 feeds a coolant 52, preferably a cooled coolinggas, in particular compressed air, through a coolant line 46 to acoolant discharge 48 which points at the external peripheral surface 42of the atomizing bell 4 behind latter's terminal zone 8 of the externalsurface, and thus this external peripheral surface 42 shall be cooled.The cold passes through the atomizing bell 4 to its front side 5. As aresult, the temperature is lowered to ambient both at the externalperipheral surface 42 of the atomizing bell 4 and on its front side 5,in order to prevent or at least reduce or delay a layer of coatingliquid being formed on these surfaces.

[0023] A cooling unit 50 to cool the coolant 52 preferably is mounteddirectly on the spray system 2 or integrated into it. Consequently thecooled coolant only moves along short paths. Preferably the coolant is acompressed gas, such as compressed air from a compressed air source 54,and is fed through a coolant feed line 56 in metered manner to thecooling unit 50 and then is blown through the coolant discharge 48,illustratively in the form of one or more nozzles, onto the atomizingbell 4.

[0024] The cooling unit 50 may contain a so-called cooling cartridge tocool the coolant 52.

1. A method for spraying coating liquids, whereby a liquid atomizer inthe form of an irrotational nozzle or preferably in the form of arotating rotary atomizing element (4) the coating liquid is sprayed ontoan object to be coated, characterized in that a front, externalsurface's terminal zone (8) of the spray system (2) discharging thespray jet of coating liquid is wetted by an accessory liquid (10) inorder to prevent or delay drying and layering of precipitated coatingliquid at the front, external surface's terminal zone (8) of the spraysystem (2).
 2. Spraying method as claimed in claim 1, characterized inthat the accessory liquid (10) is dripped onto the front, externalsurface's terminal zone (8).
 3. Spraying method as claimed in claim 1,characterized in that the accessory liquid (10) is deposited in the formof an aerosol onto the front, external surface's terminal zone (8). 4.Spraying method as claimed in one of the above claims, characterized inthat the accessory liquid (10) is deposited on the front terminal zone(8) of the external peripheral surface of the rotary atomizing element(4).
 5. Spraying method as claimed in one of the above claims,characterized in that water is used as the accessory liquid (10) whenthe coating liquid is water soluble and in that a solvent is used as theaccessory liquid when the coating liquid contains solvents.
 6. Sprayingmethod as claimed in one of the above claims, characterized in that atleast one component (4) of the spray system (2), where said component isexposed to ambient air and is streamed over by, or wetted with, coatingliquid, shall be cooled during spray coating by a cooled medium (52). 7.Spraying method as claimed in claim 6, characterized in that the coolingmedium used is a compressed gas (52), preferably compressed air, whichshall be blown on the pertinent component.
 8. A spray system for coatingliquids, containing a liquid atomizer in the form of an irrotationalnozzle or preferably in the form of a rotating rotary atomizing element(4) to spray the coating liquid onto an object to be coated,characterized in that it includes a wetting unit (18, 20) to wet bymeans of an accessory liquid (10) a front external surface's terminalzone (8) of the spray system (2) during spray coating in order toprevent or delay drying and layering of coating liquid precipitated onsaid external surface.
 9. Spray system as claimed in claim 8,characterized in that the wetting unit (18, 20) is designed to drip theaccessory liquid onto the front external surface's terminal zone (8).10. Spray system as claimed in claim 8, characterized in that thewetting unit (18, 20) is designed to deposit the accessory liquid in theform of an aerosol.
 11. Spray system as claimed in one of claims 8through 10, characterized in that the wetting unit (18, 20) is designedto wet the front, external surface's terminal zone (8) of the rotaryatomizing element (4).
 12. Spray system as claimed in one of claims 8through 11, characterized in that water is used as the accessory liquid(10) if the coating liquid is water soluble or that a solvent is used asaccessory liquid if the coating liquid contains solvents.
 13. Spraysystem as claimed in one of claims 8 through 12, characterized by acooling unit (40) to cool at least a component (4) of the spray system(2) using a cooled fluid medium during spray coating for the purpose ofpreventing or delaying curing of coating material at this component (4),said coolant being fed to said component (4).
 14. Spray coating asclaimed in claim 13, characterized in that the cooled component is therotary atomizing element (4).
 15. Spray system as claimed in either ofclaims 13 and 14, characterized in that the cooling unit (40) comprisesa coolant line (46) fitted with at least one cooling discharge (48) forcooled compressed gas, said discharge pointing at the component (4) tobe cooled in order to blow the cooled compressed gas onto saidcomponent.
 16. Spray system as claimed in claim 15, characterized bybeing fitted with a cooling unit (50) to cool the compressed gas.